The objective of the project is to characterize the optics of the human crystalline lens and to understand its contribution to accommodation and presbyopia. The project is designed to test the hypothesis that age-related changes in the gradient refractive index of the crystalline lens significantly contribute to the loss of accommodation leading to presbyopia. An age and accommodation dependent optical model of the crystalline lens with its gradient will be developed. The model will be experimentally validated using biometric measurements on isolated human and non-human primate lenses as well as experiments in a lens stretcher that simulates accommodation. There are three specific aims to the proposed study: Aim 1: Develop a gradient refractive index model that accurately predicts the power of isolated primate crystalline lenses. Aim 2: Quantify the contribution of the lens index gradient to the accommodation amplitude in a lens stretcher that simulates accommodation. Aim 3: Quantify the contribution of the lens index gradient to the age-related decrease in accommodation amplitude. The development of an age and accommodation-dependent model of the crystalline lens that incorporates the gradient will provide insight into the internal workings of the lens during accommodation and how the optical properties of the lens change with age. A better understanding of the optics of the lens will enable the design of improved surgical procedures to correct presbyopia and the optimization of vision correction procedures in general.